Blood cell counters of the Coulter type have been known for many years and were first described in the U.S. Pat. No. 2,656,508, granted Oct. 20, 1953 in the name of Wallace H. Coulter. In the basic Coulter structure, as taught by the Coulter '508 patent, a constant current is provided through a small opening, or aperture, while an electrolyte liquid solution, containing a suspension of blood cells, flows through the aperture. Each time a blood cell flows through the aperture, the internal resistance of the electrolyte within the aperture increases, thereby causing the voltage across the aperture to increase. By monitoring the voltage across the aperture, the pulse signals, which appear each time a blood cell flows through the aperture, can be detected and counted to manifest a blood cell count and volume for a fixed volume of flowing liquid.
In order to obtain the optimum operation of a Coulter type blood cell counter, it is desirable that all of the available current be applied through the aperture. This is generally accomplished by placing electrodes in the electrolyte solution at a position opposite to both sides of the aperture and providing a current from one to the other of the electrodes. The blood cell suspension or containing fluid is caused to flow through the aperture, typically, by applying a suction, or vacuum, to the downstream side of the aperture to draw the fluid through the aperture.
However, since the fluid drawn away from the aperture is an electrolyte which conducts current when a voltage difference exists from one part to another part of the flow path. Whatever current is conducted through the electrolyte, is diverted away from the aperture, thereby reducing the effectiveness of the measurements obtained from monitoring the voltage across the aperture. The voltage difference in the downstream flowpath occurs when the design of the blood cell counter causes the electrode downstream from the aperture to be at a non-reference potential and where the flow path is grounded at the point it exits the cell counter package. The reason for grounding the flow path at the point it exits the package is to prevent environmental signals, such as sixty cycle hum, due to the lights, or other environmental electrical noise, from being conducted through the electrolyte solution and causing interference in the circuitry monitoring the voltage drop across the aperture.